The present invention relates to forced air induction systems for engines and more particularly, although not exclusively, to the use of such systems for improving the performance of an internal combustion engine.
Turbochargers and mechanical superchargers are well known forms of forced air induction systems. Both systems operate by driving a compressor so as to increase the air pressure into an internal combustion engine in order to improve performance. The source of the driving force for the compressor has a significant impact on the overall performance of the engine. A turbocharger derives the energy to power the compressor from exhaust gasses from the engine via a turbine, whereas a supercharger compressor is driven directly or indirectly by the engine crankshaft.
Thus the aim of a turbocharger differs significantly from that of a supercharger. In essence, a turbocharger aims to recover energy, at least some of which would otherwise be lost, thus improving engine efficiency. A supercharger sacrifices engine efficiency by drawing power from the crankshaft in order to improve responsiveness.
The different sources of power used to drive the compressor affects the practical operation of different charge air systems.
In use, a turbocharger is required to operate in continually transient conditions over a wide range of operational speeds. However turbine design is not flexible enough to allow suitable operation of the turbocharger over the entire spectrum of engine speeds and so operation at low engine speeds is typically sacrificed in favour of improved operation of the turbocharger at greater speeds.
A further problem associated with conventional turbochargers is the boost threshold. In order to operate the compressor, it is necessary for the exhaust gas flow from the engine to be sufficient to drive the turbine at suitable rotational speeds. At low engine speeds, the exhaust gas flow may be insufficient to attain the required turbine flow rate and thus a conventional turbocharger is often unable to meet acceleration demands placed on the system until a threshold engine speed has been reached. This need for a sufficient engine exhaust pressure to operate the turbocharger results in the turbocharger being idle or ineffective at low speeds.
Given the inherent difficulties in designing a system for transient use, a turbocharger tends to be designed around ‘average’ operational conditions. Therefore turbochargers conventionally achieve optimal performance when operating in substantially steady state conditions which require a constant supply of exhaust gas. Furthermore the turbocharger inertia also causes a delay in response to variations in engine output, often referred to as turbo lag, which compromises the driveability of a turbocharged vehicle. Turbo lag may also be attributed to insufficient exhaust gas being available at the instant an acceleration demand is made by the driver.
Once the turbocharger is operational, the turbine inertia also causes the turbocharger to recover energy which is potentially unwanted by the driver upon a deceleration of the vehicle. For this reason, a wastegate is often required to effectively bypass the turbine. This results in complex systems which are prone to reliability issues and for which operational efficiency is reduced.
The connection of the supercharger compressor to the engine crankshaft results in operation of the compressor in tune with the demands made of the engine by the driver. A supercharger responds almost instantaneously to driver inputs, improving driveability even at low engine speeds, albeit at the detriment to engine efficiency.
The above problems have been well documented and a number of solutions proposed in order to supplement the operation of turbo and superchargers. US2004216458 discloses the use of an electric motor assisted turbocharger, in which the turbocharger is power assisted, particularly at low engine speeds, in order to improve responsiveness. Whilst such an arrangement, does improve driveability to some extent, it does not adequately address the problem of the turbocharger generating unwanted energy during deceleration. Thus, as is common with turbochargers, a wastegate must be provided to allow exhaust gas to bypass the turbocharger when necessary, causing potentially useful exhaust pressure to be lost. In addition, the electric motor must be synchronised with the rotation of the turbine/compressor shaft in order to be effective.
US2006032225 discloses the combined use of a supercharger and a turbocharger in conjunction with a hydraulic pump, which acts to drive the turbocharger up to an operational speed and then hold back the turbocharger to keep it at the right speed for the engine conditions. The holding back of the turbocharger to suit engine conditions results in the turbocharger efficiency being dictated by the requirements placed upon the compressor by the driver, ultimately capping the available efficiency savings.
In view of the foregoing problems, it is an object of the present invention to provide a more efficient and flexible forced induction system for an engine.